Three Anti-interference Measures From Hardware

2022-02-11 09:45:10

 

Because the original LED power supply is a linear power supply, but the linear power supply will lose a lot of energy in the form of heat when working. Linear power supply working mode, so that he from high voltage to low voltage must have a voltage device, generally are transformers, and then through rectification output DC voltage. Although bulky, heat, the advantage is that external interference is small, electromagnetic interference is small, but also easy to solve. And now use more LED switching power supply, are IN the form of PWM LED drive power supply is to let the power transistor work in on and off state. In the conduction, the voltage is low, the current is large; When switching off, the voltage is high and the current is small, so the loss generated on the power semiconductor device is also very small. The disadvantage is more obvious, electromagnetic interference (EMI) is more serious.

 

The electromagnetic compatibility of LED power supply is generally the switching circuit in the power supply. The switching circuit is one of the main interference sources of switching power supply. The switching circuit is the core of LED drive power supply. The switching circuit is mainly composed of switching tube and high frequency transformer. The DU/DT generated by it has larger pulse, wider frequency band and rich harmonics.

 

The main reason for this high frequency pulse interference is that the switch tube is loaded with the primary coil of high frequency transformer and is a inductive load. At the moment of conduction, the primary coil produces a large surge current, and a high surge peak voltage appears at both ends of the primary coil; At the moment of disconnection, due to the flux leakage of the primary coil, part of the energy is not transmitted from the primary coil to the secondary coil, and attenuated oscillations with sharp peaks are formed in the circuit, which are superplaced on the turn-off voltage to form the turn-off voltage peak. High-frequency pulses produce more emissions, and periodic signals produce more emissions. In an LED power supply system, a current spike is generated by the switching circuit, which also generates a current spike when the load current changes. This is one of the sources of electromagnetic interference.

 

In virtually all electromagnetic interference problems, the main cause is improper grounding. There are three methods of signal grounding: single point, multipoint and mixed. When the switching circuit frequency is lower than 1MHz, single point grounding method can be adopted, but it is not suitable for high frequency; In high frequency applications, multipoint grounding is preferred.

 

Mixed grounding is a method of single point grounding for low frequency and multi-point grounding for high frequency. Ground layout is key. Ground circuit of high frequency digital circuit and low level analog circuit should not be mixed. It can be said that proper printed circuit board (PCB) wiring is critical to prevent EMI.

 

In LED power supply, there are a number of intelligent LED power supply controlled by single chip microcomputer, and some LED power supply by single chip microcomputer control the duty ratio of switch circuit, the watchdog of the single chip microcomputer system for the whole run of LED power supply plays an important role in particular, due to some interference sources cannot all be isolated or in addition, once in the CPU interfere with the normal operation of the program, The combination of a reset system and software processing is an effective error-correction defense.

There are two common reset systems:

① External reset system. External watchdog circuits can be designed in-house or built with dedicated Watchdog chips. Thus, if the program system is stuck in an infinite loop, and the loop happens to have a "feed the dog" signal, the reset circuit will not be able to perform its intended function.

(2) there are more and more LED power supply with its own on a chip reset system, so users can easily use it department reset timer, however, some smart LED power supply control circuit reset instruction is too simple, that will also exist such as the infinite loop "dog" instruction, make its loss to the monitoring role.

To solve the electromagnetic interference problem of LED driving power supply, we can start from the following aspects from the hardware:

1. Reduce the interference of switching power supply itself: Soft switching technology, in the original hard switching circuit to increase inductance and capacitance elements, using the resonance of inductance and capacitance, reduce the switching process DU/DT and DI/DT, so that the switching device when the voltage drop before the current rise, or off the current drop before the voltage rise, to eliminate the overlap of voltage and current. Switching frequency modulation technology, by modulating switching frequency FC, focusing on FC and its harmonics 2FC, 3FC... In order to reduce the EMI amplitude at each frequency point. Selection of components, choose the components that are not easy to produce noise, not easy to conduct and radiation noise. Special attention is usually paid to the selection of winding components such as diodes and transformers. The fast recovery diode with low reverse recovery current and short recovery time is an ideal device for high frequency rectification of switching power supply.

 

Reasonable use of EMI filter, one of the main purposes of EMI filter, power grid noise is a kind of electromagnetic interference, it belongs to radio frequency interference (RFI), its transmitted noise spectrum is roughly 10khZ-30mhz, up to 150MHz. In general, the difference mode interference amplitude is small, low frequency, resulting in small interference; Common mode interference amplitude, high frequency, but also through the wire radiation, resulting in greater interference. In order to weaken the conducted interference, the most effective method is to install emi filters in the input and output circuits of switching power supply. LED power supply generally adopts simple single-stage EMI filter, mainly including common mode choke and filter capacitor. EMI filter can effectively suppress the electromagnetic interference of switching power adapter.

 

2. Reduce electromagnetic interference by cutting off the propagation path of interference signals: In the first case, power line interference can be filtered by power line filter. A reasonable and effective EMI filter for switching power supply should have a strong suppression effect on the differential mode and common mode interference on the power line. PCB is the support part of circuit elements and components in LED power supply system, it provides the electrical connection between circuit elements and components. With the rapid development of electronic technology, PCB density is getting higher and higher.

The design of PCB has great influence on the electromagnetic compatibility of LED power supply system. Practice has proved that even if the circuit schematic design is correct, the printed circuit board design is not correct

 

The reliability of the LED power system is adversely affected. PCB anti-interference design mainly includes PCB layout, wiring and grounding, its purpose is to reduce the electromagnetic radiation of PCB and the crosstalk between circuits on PCB. In addition, the frequency of ac sound caused by electromagnetic interference of transformers is generally about 50HZ, while the ac sound caused by improper grounding wiring is about 100HZ due to the frequency doubling of the rectifier circuit, which can be detected by careful differentiation. Therefore, in the design of printed circuit board, we should pay attention to the correct method, comply with the general principle of PCB design, and should meet the design requirements of anti-interference.

3. Take the initiative to significantly enhance the anti-interference ability of the disturbance of body: type in the LED power supply system in/output is also interference sources of the transmission line, and receive radio frequency interference signal source, picking generally must take effective measures: when we design the necessary/common-mode differential mode suppression circuit, as well as a filter and the electromagnetic shielding measures to reduce interference in. As far as conditions permit, various isolation measures (such as photoelectric isolation or magnetoelectric isolation) are adopted to interrupt the propagation of interference. Lightning protection measures, outdoor LED power supply system or from the outdoor to introduce indoor power lines, signal lines, to consider the system lightning protection issues. Common lightning protection devices include gas discharge tubes and Transient Voltage Suppression (TVS). Gas discharge tube is when the voltage of the power supply is greater than a certain value, usually tens of V or hundreds of V, gas breakdown release

Electricity, directing a strong impulse from the power line to the earth. TVS can be regarded as two parallel Zener diodes in opposite directions, which turn on when the voltage at both ends is higher than a certain value. It is characterized by transient currents of hundreds or thousands of A.

 

Through this paper, we can summarize the main control technologies for LED power EMC/EMI: circuit measures, EMI filtering, component selection, shielding and printed circuit board anti-interference design. If you can solve these problems correctly and reasonably, it is not a problem to pass the national certification smoothly! The above is some analysis of LED drive power design, I hope to help you.